This invention relates to a process for removing vanadium residues from solutions of .alpha.-olefin homopolymers and copolymers with hydrocarbon monomers, which have been prepared in the presence of vanadium-based catalysts.
Homopolymerization and copolymerization of many .alpha.-olefins are known. For the purposes of the present disclosure, the term "polymerization" includes both homo- and copolymerization, and the term "polymer" includes both homo- and copolymers. Typical commercial polymers include polyethylene, EPDM rubber, and ethylene/propylene dipolymers (EPM rubber). In many .alpha.-olefin polymerization processes, a vanadium-based coordination catalyst is used. Coordination catalysts are well known in the polymer art as Ziegler catalysts. Vanadium-based catalysts are made by combining a vanadium compound, preferably one soluble in hydrocarbons, and a organometallic reducing compound of a metal of Groups I-III of the Periodic Table, preferably aluminum.
Polymerization is normally carried out in a liquid phase, often in solution in a saturated hydrocarbon, most often in a continuous system. The reactor effluent containing the polymer usually also contains various impurities, for example, spent catalyst residues and unchanged monomer. It is important to remove the vanadium residues from the polymer solution before the polymer is recovered because such residues, if present in the polymer, adversely affect its heat stability. Polymer can be isolated from solution by phase decantation, i.e., by heating the solution to a temperature at which the solution separates into two phases: a polymer-poor phase and a polymer-rich phase, from which polymer is recovered by flashing off residual solvent. This procedure, described in U.S. Pat. No. 3,726,843 to Anolick et al., produces a polymer containing a significant amount of vanadium impurities unless catalyst reactivation is employed during the polymerization. It is desirable to minimize the amount of vanadium residue in the polymer solution before phase decantation occurs. Vanadium removal is, similarly, usually desirable from polymer solutions which are to be subjected to steam vaporization, for example, as taught in U.S. Pat. No. 3,750,736 to Batt et al.
Various methods of removing vanadium from the polymers are known. For example, U.S. Pat. No. 3,271,372, to S. W. Caywood, Jr., discloses a process wherein an alcohol is added to an ethylene copolymer slurry in methylene chloride to disperse the granular particles of the copolymer and the copolymer is separated from the slurry. U.S. Pat. No. 3,337,514, to Knabeschuh et al., requires contacting a solution of an .alpha.-olefin copolymer with steam, then with aqueous mineral acid, then with water under turbulent conditions, and separating the copolymer solution from the aqueous phase. When water is mixed with a polymer solution, it is necessary to avoid emulsification because trapped water can lead to incomplete vanadium removal. Furthermore, emulsification can disturb the equilibrium in the above-mentioned phase decantation process. Other techniques are available but all have shortcomings in that complicated installations are required, and that normally other chemical compounds are introduced which then must be separated.
There is a need for a simple and efficient process for removing vanadium residues from .alpha.-olefin hydrocarbon polymer solutions, and especially a process suitable for continuous vanadium removal.